Starfish, along with other echinoderms such as sea urchins, sand dollars, and sea cucumbers, are unique creatures that lack the architecture for an actual body. According to a new analysis of their gene expression, starfish and other echinoderms are essentially just mobile heads that sprouted the ability to crawl.
Their body symmetry is usually fivefold, rather than the bilateral ‘left-right’ symmetry we see in most creatures. This strange symmetry means that their bodies are difficult for us to figure out. In their bilateral relatives, the body is divided into a head, trunk, and tail. But just looking at a starfish, it’s impossible to see how these sections relate to the bodies of bilateral animals.
The oldest known starfish on the fossil record predates the earliest known dinosaurs by over 200 million years, so whatever it is they’re doing, they’re very, very good at it.
Starfish are brainless, bloodless, digest their food externally, and regenerate body parts, sometimes into whole other new starfish. They are essentially just mobile heads that sprouted the ability to crawl.
The researchers undertook a molecular study to find out where echinoderms fit in the deuterostome superphylum, a large group of animals that includes both vertebrates and echinoderms. Since deuterostomes evolved from a common ancestor, many millions of years ago, the researchers thought they may be able to track the way echinoderms evolved.
Their study was conducted on a species of sea star called Patiria miniata, or bat stars. They used RNA tomography and a technique called in situ hybridization, in which scientists localize precise DNA and RNA sequences in a tissue sample. They used this information to create a three-dimensional map of gene expression in the body of the sea star as it grew.
They investigated a range of transcription factors that are in bilateral animals involved in the front-to-back development of the creature’s body, a process known as anterior-posterior patterning. These genes were found in the sea star; they help develop the arms, from about the mid-region down to the tip.
But there was a glaring omission. In other deuterostomes, there is a set of genes that helps develop the trunk of the body.
When we compared the expression of genes in a starfish to other groups of animals, like vertebrates, it appeared that a crucial part of the body plan was missing,” says evolutionary biologist Jeff Thompson of the University of Southampton in the UK.
“The genes that are typically involved in the patterning of the trunk of the animal weren’t expressed in the ectoderm. It seems the whole echinoderm body plan is roughly equivalent to the head in other groups of animals.”
The evolution of echinoderms has been difficult to understand, just based on the way their (head-) bodies are shaped. The new research suggests that, once upon a time, echinoderms may have had the tools to develop a body, but dropped it somewhere along their long history.
In their defense, it seems to have worked out pretty well for them.
But the new findings give scientists new tools for understanding why they are that way, and interpreting any fossils that emerge from eons past.
“Our research tells us the echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures,” Thompson says.
“As someone who has studied them for the last ten years, these findings have radically changed how I think about this group of animals.”